- create a helper function __updateProfileEnvironment in the
  KIWIImageCreator code to reduce code duplication
- prepareBootImage and createBootImage always run in a sequence
  thus it's not required to read in the boot image description
  twice. The XML description for the boot (initrd) image is now
  read in once in boot prepare and just re-used next in the boot
  create step. This also simplifies the handling of inherited
  data from the system into the boot image